Deficiency of the Nrf1 and Nrf2 Transcription Factors Results in Early Embryonic Lethality and Severe Oxidative Stress

Abstract

Nrf1 and Nrf2 are members of the CNC family of bZIP transcription factors that exhibit structural similarities, and they are co-expressed in a wide range of tissues during development. Nrf2 has been shown to be dispensable for growth and development in mice. Nrf2-deficient mice, however, are impaired in oxidative stress defense. We previously showed that loss of Nrf1 function in mice results late gestational embryonic lethality. To determine whether Nrf1 and Nrf2 have overlapping functions during early development and in the oxidative stress response, we generated mice that are deficient in both Nrf1 and Nrf2. In contrast to the late embryonic lethality in Nrf1 mutants, compound Nrf1, Nrf2 mutants die early between embryonic days 9 and 10 and exhibit extensive apoptosis that is not observed in the single mutants. Loss of Nrf1 and Nrf2 leads to marked oxidative stress in cells that is indicated by elevated intracellular reactive oxygen species levels and cell death that is reversed by culturing under reduced oxygen tension or the addition of antioxidants. Compound mutant cells also show increased levels of p53 and induction of Noxa, a death effector p53 target gene, suggesting that cell death is potentially mediated by reactive oxygen species activation of p53. Moreover, we show that expression of genes related to antioxidant defense is severely impaired in compound mutant cells compared with single mutant cells. Together, these findings indicate that the functions of Nrf1 and Nrf2 overlap during early development and to a large extent in regulating antioxidant gene expression in cells.Reactive oxygen species (ROS) 1 are generated during aerobic respiration and normal metabolic processes, and they are also byproducts of metabolism of a wide range of environmental agents (1). High levels of ROS are detrimental to the cell, since they react readily with intracellular molecules, causing cell injury and death (1). Cells are equipped with a variety of defense mechanisms that work in parallel or in sequence to minimize ROS levels. These defenses include enzymes that are involved in ROS metabolism and biotransformation of xenobiotics (2). Examples of these enzymes include superoxide dismutases, glutathione peroxidases, thioredoxins, and heme-oxygenases as well as phase 2 enzymes, such as glutathione S-transferases. In addition to enzymatic defenses, cells are also equipped with molecules such as glutathione, metallothioneins, and ferritins that scavenge ROS and metal ions. Basal and inducible expression of a number of these antioxidant defense genes are mediated in part by a cis-acting DNA element known as the antioxidant response element (3-6). Activation through the antioxidant response element (ARE) appears to be driven by conditions that promote intracellular oxidative stress (5,7,8). A number of different transcription factors including basic leucine zipper proteins, AP-1, have been shown to bind the ARE (3, 9, 10).Recent studies implicate the CNC subfamily of bZIP proteins in mediating ARE function (11). Members o...

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